She said herself in this movie that she could only do limited things with her telekinesis. Her power grows in the sequels and there's a plot point in the third film explaining why it needed to.
She had been training with her powers for years. She should have known their limitations and have had no expectation to be able to free herself from heavy metal restraints if the best she seemed to be able to do is support the weight of a human body.
As you note, she is not overly useful in combat here. She has very limited use of her full abilities, which only start to come out in "X2."
Also, if you watch closely at the climax of X1, when they destroy Magneto's machine, Jean is hit by a wave of energy. This may be what amplifies her powers and leads into her lack of control of them in X2.